R. Baboian and G. S. Haynes have written a paper entitled "Galvanic Corrosion of Ferritic Stainless Steels in Sea Water" which is published in "Corrosion in Natural Environments", ASTM STP 558, American Society for Testing and Materials, 1974, pages 171 through 184. The paper describes the corrosion of stainless steels in sea water solutions, and particularly notes in FIG. 3, page 175 that when the potential on the stainless steel is more active than a critical potential, designated E.sub.p, no pitting corrosion of the stainless steel occurs.
A similar figure is in an article by J. F. Bates entitled "Cathodic Protection to Prevent Crevice Corrosion of Stainless Steels in Halide Media" which appears in "Corrosion", Volume 29, pages 28 through 32, January, 1973.
When describing the invention, ASTM Standard G3-74, "Standard Recommended Practice for Conventions Applicable to Electro-Chemical Measurements in Corrosion Testing" will be followed.
Protection of stainless steel in the presence of chloride ions from pitting corrosion comprises the steps of inducing cathodic currents in the stainless steel in conjunction with active voltages. The active voltages are everywhere on the stainless steel greater than a predetermined minimum voltage which preferably is at least -500 millivolts with respect to a silver-silver chloride reference electrode. Causing the induced active voltages to exceed -700 millivolts unnecessarily substantially increases the current requirements and the energy used in the protection. Further, when the current flow is caused by a sacrificial anode, the anode is unnecessarily rapidly used.
Prior to this invention, protection of the interior of stainless steel tubes from pitting corrosion was believed impossible by the use of an exterior anode beyond about three to five diameters into the tube.